Isocyanates are widely used as raw materials of such products as polyurethane foam, paints, adhesives and the like. The main industrial production process of isocyanates involves reacting amine compounds with phosgene (phosgene method), and nearly the entire amount of isocyanates produced throughout the world are produced according to the phosgene method. However, the phosgene method has numerous problems.
Firstly, this method requires the use of a large amount of phosgene as the raw material. Phosgene is extremely toxic and requires special handling precautions to prevent exposure of handlers thereof, and also requires special apparatuses to detoxify waste.
Secondly, since highly corrosive hydrogen chloride is produced in large amounts as a by-product of the phosgene method, in addition to requiring a process for detoxifying the hydrogen chloride, in many cases hydrolytic chlorine is contained in the isocyanates produced. Consequently, in the case of using isocyanates produced by the phosgene method, the isocyanates may have a detrimental effect on the weather resistance and heat resistance of polyurethane products.
On the basis of this background, a process for producing isocyanate compounds has been sought that does not use phosgene. One example of a method for producing isocyanate compounds without using phosgene that has been proposed involves thermal decomposition of carbamic acid esters. Isocyanates and hydroxy compounds have long been known to be obtained by thermal decomposition of carbamic acid esters (see, for example, Non-Patent document 1). The basic reaction is illustrated by the following formula:R(NHCOOR′)a→R(NCO)a+aR′OH  (1)(wherein R represents an organic residue having a valence of a, R′ represents a monovalent organic residue, and a represents an integer of 1 or more).
On the other hand, thermal decomposition of carbamic acid esters is susceptible to the simultaneous occurrence of various irreversible side reactions such as thermal denaturation reactions undesirable for carbamic acid esters or condensation of isocyanates formed by the thermal decomposition. Examples of these side reactions may include a reaction in which urea bonds are formed as represented by the following formula (2), a reaction in which carbodiimides are formed as represented by the following formula (3), and a reaction in which isocyanurates are formed as represented by the following formula (4) (see Non-Patent documents 1 and 2).

Note that in the above formulas, R and R′ represent groups such as aliphatic alkyl groups or aromatic alkyl groups.
In addition to these side reactions leading to a decrease in yield and selectivity of the target isocyanate, in the production of polyisocyanates in particular, these reactions may make long-term operation difficult as a result of, for example, causing the precipitation of polymeric solids that clog the reaction vessel.
Various processes have been proposed thus far for the production of isocyanates without using phosgene.
According to the description of Patent document 1, aliphatic diurethane and/or alicyclic diurethane and/or aliphatic polyurethane and/or alicyclic polyurethane are obtained by reacting aliphatic primary diamine and/or alicyclic primary diamine and/or aliphatic primary polyamine and/or alicyclic primary polyamine in the presence of an O-alkyl carbamate and alcohol, in the presence or absence of a catalyst at a temperature of from 160 to 300° C. such that the ratio of amine NH2 groups to carbamate to alcohol is 1:0.8 to 10:0.25 to 50, and by removing the ammonia formed as necessary. The resulting diurethane and/or polyurethane can be converted to the corresponding diisocyanate and/or highly functional polyisocyanate as necessary. Details of the reaction conditions of the thermal decomposition are not described in the applicable patent document.
According to Patent document 2, aromatic diisocyanates and/or polyisocyanates are produced by going through the following two steps. More specifically, in the first step, an aromatic primary amine and/or aromatic primary polyamine are reacted with an O-alkyl carbamate in the presence or absence of a catalyst and in the presence or absence of urea and alcohol to form an aryl diurethane and/or aryl polyurethane followed by removal of the ammonia formed as necessary. In the second step, an aromatic isocyanate and/or aromatic polyisocyanate are obtained by thermal decomposition of the aryl diurethane and/or aryl polyurethane.
Other publications contain descriptions relating to the partial substitution of urea and/or diamine a carbonyl-containing compound such as N-substituted carbamate and/or dialkyl carbonate, or by mono-substituted urea, di-substituted urea, mono-substituted polyurea or di-substituted polyurea (see Patent document 3, Patent document 4, Patent document 5, Patent document 6 and Patent document 7). Patent document 8 describes a process for producing aliphatic O-aryl urethane by reacting (cyclic) aliphatic polyamines with urea and aromatic hydroxy compounds.
Several processes are known for forming the corresponding isocyanate and alcohol by thermal decomposition of the (cyclic) aliphatic, and particularly the aromatic monourethanes and diurethanes, examples of which may include a process carried out at a high temperature in a gaseous phase, and a process carried out under comparatively low temperature conditions in a liquid phase. In these processes, however, since there are cases in which, for example, the reaction mixture forms precipitates, polymeric substances and closed compounds in the reaction vessel and recovery apparatus due to the occurrence of side reactions as previously described, or these substances form substances that adhere to the walls of the reaction vessel, economic efficiency is poor in the case of producing isocyanates over a long period of time.
Thus, chemical methods, such as the use of a special catalyst (see Patent document 9 and Patent document 10) or a catalyst combined with an insert solvent (see Patent document 11) are disclosed for improving yield during thermal decomposition of urethane.
For example, Patent document 12 describes a process for producing hexamethylene diisocyanate involving thermal decomposition of hexamethylene diethyl urethane in the presence of dibenzyl toluene used as a solvent and in the presence of a catalyst mixture containing methyl toluene sulfonate and diphenyl tin dichloride. However, since there is no detailed description of production of the starting components, isolation or purification and arbitrary recovery of the solvent and catalyst mixture, the economic effects of this process were unable to be assessed.
According to the process described in Patent document 13, urethane can be easily decomposed to isocyanate and alcohol in a carbon-containing fluidized bed without using a catalyst. In addition, according to the description of Patent document 14, hexamethylene dialkyl urethane can be decomposed in a gaseous phase at a temperature exceeding 300° C. in the presence or absence of a gas-permeable packaging material composed of, for example, carbon, copper, bronze, steel, zinc, aluminum, titanium, chromium, cobalt or quartz, resulting in the formation of hexamethylene diisocyanate.
According to the description of Patent document 14, the process is carried out in the presence of a hydrogen halide and/or hydrogen halide donor. However, this process is unable to achieve a yield of hexamethylene diisocyanate of 90% or more. This is because the decomposition product partially rebonds resulting in the formation of urethane bonds. Thus, purification of the hexamethylene diisocyanate by distillation is still required, and there are numerous cases in which yield loss increases.
Moreover, Patent document 15 discloses that monocarbamates can be advantageously decomposed at high yield without using a solvent under reduced pressure and/or in the presence or absence of a stabilizer and at a comparatively low temperature. The decomposition products (monoisocyanates and alcohol) are removed by distillation from a boiling reaction mixture and captured separately by fractional condensation. A method for partially removing the reaction mixture is generically described in order to remove by-products formed during thermal decomposition. Thus, although it is possible to remove by-products from the bottom of the reaction vessel, the problem of the case of substances adhering to the walls of the reaction vessel as previously described remains, and problems regarding long-term operation are unresolved. In addition, there is no description regarding the industrial use of the removed residual substances (containing large amounts of useful components).
According to the description of Patent document 16, thermal decomposition of aliphatic, alicyclic or aromatic polycarbamates is carried out at from 150 to 350° C. and from 0.001 to 20 bar, in the presence of an inert solvent, and in the presence or absence of a catalyst and assistant in the form of hydrogen chloride, organic acid chloride, alkylation agent or organic tin compound. By-products formed can be removed continuously from the reaction vessel together with the reaction solution, for example, and corresponding amounts of fresh solvent or recovered solvent are added simultaneously. Examples of disadvantages of this process may include a decrease in the space time yield of polyisocyanate due to the use of a circulating solvent, and a large energy requirement, including recovery of the solvent. Moreover, since the assistant used is volatile under the reaction conditions, contamination of the decomposition products can occur. In addition, since there is a large amount of residual substances formed relative to the polyisocyanate formed, there is some doubt regarding economic efficiency and reliability as an industrial process.
Patent document 17 describes a process for continuous thermal decomposition of a carbamate supplied along the inner walls of a tubular reaction vessel in the form of a liquid in the presence of a high boiling point solvent, an example of which may include an alicyclic diurethane in the form of 5-(ethoxycarbonylamino)-1-(ethoxycarbonylaminomethyl)-1,3,3-trimethylcyclohexane. This process has the shortcomings of low yield and low selectivity during production of (cyclic) aliphatic diisocyanates. In addition, there is no description of a continuous method accompanying recovery of rebonded or partially decomposed carbamates, nor is there any mention of post-treatment of solvent containing by-products and catalyst.
According to the description of Patent document 18, a circulating process is disclosed for producing (cyclic) aliphatic diisocyanates by conversion of the corresponding diamine to diurethane followed by thermal decomposition of the urethane. This process minimizes decreases in yield by recirculating the product of the urethane decomposition step following reaction with alcohol to an urethanation step. By-products that are unable to be recirculated are removed by separating the by-products by distilling a mixture of the urethanation products, and in this case, residues of no value are formed in the form of bottom products, and all components having a comparatively low boiling point, including diurethane, are removed from the top of the column. However, this process has the shortcoming of using a large amount of energy. This is because, in addition to requiring all diurethanes to be evaporated in the presence of a catalyst, the diurethanes must be evaporated at a temperature level within a range of the decomposition temperature of urethane. Isocyanate groups formed in useful products react with residual urethane, frequently resulting in the formation of comparatively high molecular weight by-products that decrease yield.
According to the description of Patent document 19, a process is disclosed whereby worthless by-products are partially removed outside the system prior to carrying out thermal decomposition of polyurethane. The shortcoming of this process is a decrease in the yield of isocyanate since polyurethane ends up being contained in the by-products partially removed outside the system. In addition, although components that do not undergo thermal decomposition present in a reaction mixture obtained in the thermal decomposition step of polyurethane and containing unreacted polyurethane, high boiling point oligomers, and other worthless by-products that are able to be reused are separated and continuously removed from the thermal decomposition apparatus and recirculated to the urethanation step following reaction with alcohol either directly or as necessary in an attempt to increase the yield of isocyanates, recirculated high boiling point oligomers present in the system during the urethanation step may precipitate in the urethanation reaction vessel and gradually accumulate on the walls of the reaction vessel, thereby impairing operation over a long period of time.
In addition, according to the description of Patent document 20, isocyanates are produced by continuous thermal cleavage decomposition of carbamic acid ester using a process in which a reaction medium containing carbamic acid ester is heated so that a biphasic mixture is formed having a gas volume of greater than 50%, the gaseous phase is continuously discharged from the reaction vessel, and the liquid phase is continuously discharged from the reaction vessel. In this process as well, although components that do not undergo thermal decomposition present in a reaction mixture containing unreacted polyurethane, high boiling point oligomers, and other worthless by-products that are able to be reused are separated and continuously removed from the thermal decomposition apparatus and recirculated to the urethanation step following reaction with alcohol either directly or as necessary in an attempt to increase the yield of isocyanates, similar to the process described above, recirculated high boiling point oligomers present in the system during the urethanation step may precipitate in the urethanation reaction vessel and gradually accumulate on the walls of the reaction vessel, thereby impairing operation over a long period of time.
Patent document 21 discloses a process for carrying out thermal decomposition by evaporating methyl urethane, obtained by reacting dimethyl carbonate and amine in the presence of a basic catalyst followed by introducing into a thermal decomposition reaction vessel. Although unevaporated components are removed from the bottom of the evaporator during evaporation of methyl urethane, since methyl urethane ends up being contained in the removed components, this process has the shortcoming of causing a decrease in the yield of isocyanate. In addition, thermal denaturation of methyl urethane also tends to occur easily since methyl urethane vapor is transferred at a high temperature.    Patent document 1: U.S. Pat. No. 4,497,963    Patent document 2: U.S. Pat. No. 4,290,970    Patent document 3: U.S. Pat. No. 4,388,238    Patent document 4: U.S. Pat. No. 4,430,505    Patent document 5: U.S. Pat. No. 4,480,110    Patent document 6: U.S. Pat. No. 4,596,678    Patent document 7: U.S. Pat. No. 4,596,679    Patent document 8: European Patent Publication No. 0320235    Patent document 9: U.S. Pat. No. 2,692,275    Patent document 10: U.S. Pat. No. 3,734,941    Patent document 11: U.S. Pat. No. 4,081,472    Patent document 12: U.S. Pat. No. 4,388,426    Patent document 13: U.S. Pat. No. 4,482,499    Patent document 14: U.S. Pat. No. 4,613,466    Patent document 15: U.S. Pat. No. 4,386,033    Patent document 16: U.S. Pat. No. 4,388,246    Patent document 17: U.S. Pat. No. 4,692,550    Patent document 18: European Patent No. 0355443    Patent document 19: U.S. Pat. No. 5,386,053    Patent document 20: Japanese Patent No. 3238201    Patent document 21: U.S. Pat. No. 5,315,034    Non-Patent document 1: Berchte der Deutechen Chemischen Gesellschaft, Vol. 3, p. 653, 1870    Non-Patent documents 2: Journal of American Chemical Society, Vol. 81, p. 2138, 1959